The present invention relates to a process for the purification of hydrogen-containing silanes and siloxanes and more particularly the present invention relates to a process for the purification of hydrogen-containing silanes and hydrogen-containing siloxanes having no phenyl substituent groups so as to remove from said silanes and siloxanes all impurities which would poison a platinum catalyst.
The SiH-olefin platinum-catalyzed reactions are well known. There is also produced a room temperature vulcanizable silicone rubber product which is formed through an SiH-olefin, platinum-catalyzed reaction. Such product generally comprises a hydride polysiloxane which is normally packaged in a separate package and a second component which is a mixture of a vinyl-containing polysiloxane, filler and a platinum catalyst. When the two components are mixed, the hydrogen of the hydrogen-containing siloxane in the presence of the platinum catalyst adds to the olefinic group in the vinyl-containing polysiloxane to cross-link and cure the composition at room temperature to a silicone elastomer. As can be appreciated, it is vital in such processes and compositions that the platinum catalyst not be poisoned.
Accordingly, if the hydride siloxane or the vinyl polysiloxane component contains impurities which will poison the platinum catalyst, then the composition will not cure when the two components are mixed together. There also can be present impurities in the composition which will retard or partially poison the platinum catalyst, thus requiring additional amounts of platinum catalyst to carry out the reaction, thus increasing the expense of the product. It has been found that normally such impurities come into the product with the hydrogen-containing polysiloxane. At any rate, it is desirable to obtain the hydrogen-containing polysiloxane in as pure a form as possible and without containing impurities which can poison the platinum catalyst since if that is the case, then the SiH-olefin reaction will proceed without difficulty and with the use of only a very small amount of the platinum catalyst. Normally, such hydrogen-containing polysiloxanes are obtained by hydrolyzing in water, the appropriate dichlorosilane with the appropriate amount of hydrochlorosilane chain-stoppers to obtain a low molecular weight linear polysiloxane polymer with the appropriate hydrogen substitution. Such a polymer can then be utilized as a cross-linking agent in SiH-olefin, platinum-catalyzed compositions to produce room temperature vulcanizable silicone rubber compositions. Accordingly, then a hydrogen-containing polysiloxane utilized in such SiH-olefin reactions must be as pure as possible and contain as little as possible of impurities as is the case with the hydrogen-containing chlorosilane that is produced therefrom. Accordingly, it is desired to have the appropriate hydrogen-containing chlorosilane as pure as possible and as free as possible of impurities. It should be noted that it is desired that hydrogen containing chlorosilanes in another respect be as free of impurities that poison platinum catalysts since such hydrogen-containing silanes, for instance methyldichlorosilane, are utilized to produce many intermediates which are further reacted to produce silicone elastomers and silicone fluids. For instance, dimethylhydrogenchlorosilane is reacted with vinyl acetonitrile in the presence of a platinum catalyst to produce the corresponding nitrile chlorosilane which chlorosilane is then hydrolyzed selectively or otherwise to produce the appropriate nitrile substituted silicone fluid and in which fluid the nitrile group can be further hydrolyzed into a carboxy group. Such materials can find use, for instance, as surfactants or as intermediates for producing polysiloxane-polyether copolymers which are very useful as polyurethane foam surfactants.
Another reaction which is important with a hydrogen-containing polysiloxane is the reaction of hydrogen-containing polysiloxane low molecular weight fluid with alpha-methylstyrene or with hexene to produce in the presence of a platinum catalyst the appropriate addition product. Again such substituted polysiloxane materials are useful as silicone paintable water-repellent fluids.
A more important and prominent process of chlorosilanes is the reaction of methyldichlorosilane with 3,3,3-trifluoropropene to produce 3,3,3-trifluoropropylmethyldichlorosilane. This reaction is also carried out in the presence of a platinum catalyst. Such dichlorosilane addition products after they are formed are hydrolyzed in water to produce a mixture of low molecular weight linear polysiloxanes, and a mixture of cyclosiloxanes in which the predominate species in the cyclosiloxanes is the cyclictrisiloxanes and cyclotetrasiloxanes.
By adding an alkali metal hydroxide catalyst to the hydrolyzate, such as, potassium hydroxide, in the appropriate amounts and cracking the hydrolyzate at elevated temperatures, that is, temperatures above 150.degree. C., there can be preferentially distilled overhead cyclotrisiloxanes in large yields. Such cyclotrisiloxanes may then be taken and there may be added to them a small amount of alkali metal hydroxide catalyst such as, potassium hydroxide at a concentration of anywhere from 10 to 500 parts per million and there is also added to the mixture the necessary amounts of low molecular weight linear polysiloxane chain-stopper such as, for instance, hexylmethyldisiloxane. The resulting mixture is heated at elevated temperatures, that is, temperatures above 150.degree. C. to convert the cyclotrisiloxane to a linear polysiloxane polymer having 3,3,3-trifluoropropyl substituent groups. Such a polymer with the appropriate amount of filler and peroxide curing catalyst can be cured at elevated temperatures to form a silicone elastomer with excellent solvent resistance. Accordingly, as can be envisioned, the reaction of the methyldichlorosilane with 3,3,3-trifluoropropene in the presence of a platinum catalyst is a very important step in the carrying out of the preparation of such solvent resistant silicone elastomers. However, it has been found that the reaction at times would not initiate, even after prolonged waiting or contact between the methyldichlorosilane and the 3,3,3-trifluoropropene. It was found on occasions that the reaction would not initiate even with heating of the mixture above room temperature and also even with other steps being taken to initiate the reaction. Such failures of the reaction to initiate many times resulted in the loss of materials, that is, the materials have to be discarded and new materials had to be used, and of course there was the wasted time and labor. This would unduly increase the cost of the overall process. It was found that at times, that after the reaction had failed to initiate, the reaction could be initiated by the addition of additional platinum catalyst. However, one difficulty with this addition was to increase the cost that was imputed to the process. Further, many times there would be a large amount of olefin present in the reaction mixture because of the continuous feeding of olefin to the hydrogen-containing siloxane in the reaction vessel as is normal during such reactions, and as a result the reaction might initiate suddenly and many times violently, thus creating a safety hazard. At any rate, it was determined that the reason for this inability to initiate the reaction, in some cases, was due to the presence of impurities in the hydrogen-containing polysiloxane from the process by which it was made which impurities were not removed by the distillation procedure utilized to purify the hydrogen-containing silanes. Hydrogensilanes are formed in the basic reaction of methylchloride with silicon metal in the presence of a copper catalyst at elevated temperatures, that is, temperatures in the neighborhood of 300.degree. C., or above where there is formed a host of methylchlorosilanes and hydrogen-containing silanes. The specific chlorosilanes are then separated from the mixture by fractional distillation. It has been found that by such fractional distillation and even by repeated distillation purification procedures that in some cases the platinum-poisoning impurity was not removed from the hydrogen-containing silane.
Accordingly, it was highly desirable to find the process to purify such hydrogen-containing silanes and to remove the impurities which would poison the platinum catalyst. It should be noted that at this time it is not known for certain what these impurities that poison the platinum catalyst are. It is felt that possibly the impurity is sulfur since 5 parts per million of sulfur will poison 100 parts per million of platinum. However, sulfur is very hard to analyze for in hydrogen-containing silanes and as such, at this time, what the impurities are that would poison the platinum catalyst and hydrogen-containing silanes is not known for certain. However, at any rate, various purification processes were tried to remove such impurities from the hydrogen-containing silanes and siloxanes, all of which met with failure. Such purification processes were basically distillation procedures. There should also be noted the process of the patent application of Harry R. McEntee, entitled "Process for Removing Biphenyls from Chlorosilanes", Ser. No. 828,367, filed on Aug. 29, 1977. This patent application deals with the removal of chlorinated biphenyls from streams of silane and siloxanes containing aromatic substitution, by contacting the streams of silanes and siloxanes with an adsorbent bed of molecular sieves and more preferably an adsorbent bed made of activated carbon so as to absorb the biphenyl impurities into the adsorbent bed. However, prior to the present time, such a process had never been tried on hydrogen-containing silanes and siloxanes for removing impurities therefrom which impurities would poison the platinum catalyst in SiH-olefin addition reactions.
Accordingly, it is one object of the present invention to provide for a process for removing impurities which would poison a platinum catalyst from a stream of hydrogen-containing silanes and hydrogen-containing siloxanes.
An additional object of the present invention is to remove impurities from hydrogen-containing silanes and hydrogen-containing siloxanes which impurities would poison the platinum catalyst by contacting the hydrogen-containing silanes and hydrogen-containing siloxanes with an adsorbent bed constructed of molecular sieves.
It is still an additional object of the present invention to provide for a process for removing impurities from hydrogen-containing silanes and hydrogen-containing siloxanes having no aromatic substitution, which impurities would poison the platinum catalyst in an SiH-olefin platinum-catalyzed reaction.
It is yet an additional object of the present invention to provide for a process for removing impurities from hydrogen-containing silanes and hydrogen-containing siloxanes having no aromatic substitution by contacting the hydrogen-containing silanes and hydrogen-containing siloxanes with an adsorbent bed constructed from charcoal wherein the impurities are such that they would poison a platinum catalyst in an SiH-olefin, platinum-catalyzed reaction.
These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.